The present invention relates generally to a galvanic primary cell of the type having a thickened alkaline electrolyte, and in particular to a galvanic primary cell of this type which contains an inner negative electrode comprised of a pasty zinc powder, an outer positive manganese dioxide or silver oxide electrode in the form of a ring-shaped cylinder, and an interposed separator, in a concentric arrangement within a cup-shaped metal housing.
During the assembly of such a round alkaline cell, pressed, ring-shaped parts comprised of a manganese dioxide/graphite mixture or silver oxide are first introduced as a stack into a cylindrical plate housing, and pressed to form a tubular cathode. The separator is then tightly fit into this tube in thin nonwoven layers or films, in the form of a sleeve or overlapping layers. The remaining inner space is then filled, either partially or completely, with a mixture of zinc powder, a thickener and an electrolyte (preferably potassium hydroxide), to form the cell's anode. The conductor for the anode is preferably a brass or copper nail which is concentrically received within the zinc powder mixture, and to which is soldered the cell's cap. The cap is isolated from the plate housing by a ring-shaped plastic element, which also serves as a seal.
Generally, the zinc powder and the aqueous alkali solution are separately introduced into the cell, with the result that the aqueous alkali solution tends to collect either above or below the zinc powder. In such cases, it has been found that the flat and uniform distribution of the resulting paste in all directions on the inside of the separator is not assured. Even if the zinc powder is initially stirred with the alkali solution and thickener to form the paste, intimate mixing with the aqueous alkali solution within the cell will still not take place. In either case, the zinc powder remains mobile, and the electrical state and discharge parameters may therefore change considerably during any subsequent shaking of the cell.
This lack of homogeneity of the zinc paste is especially troublesome in connection with cells which are intended to operate at high working temperatures, such as the primary cell described in DE-OS No. 33 37 570. Because of the risks of bursting such cells as a consequence of significantly increasing internal pressures, it has proven advisable to fill the anode space to only about 2/3 of its actual capacity to provide a collection space for the gases being generated (e.g., a few cm.sup.3, depending on the cell size). This assumes a rate of H.sub.2 generation for a "baby"- type cell of about 1 cm.sup.3 /hour at 163.degree. C.
Contrary to conventional alkaline round cells, wherein the anode space is practically completely filled, the above-described reduced filling which is indicated for alkaline cells having a thickened electrolyte is relatively unpredictable, resulting in the sharp inflections of the discharge curve which can sometimes occur during discharges at high temperatures. Moreover, the circular filling of the anode space tends to stratify horizontally, such that the zinc (being the heaviest component) essentially occupies the lower third of the anode space. The middle third is occupied by the aqueous electrolyte, while the top third is a hollow space.
DE-PS No. 1,201,436 attempts to remedy this situation by attempting restratification of the zinc as a homogeneous distribution opposite the cathode surface by centrifugation. With existing cell designs, this did not lead to the desired result because the brass nail which serves as the negative conductor tended to lose contact with the zinc. Moreover, the zinc layer was unstable and tended to collapse during shaking.